1. Field of the Invention
The present invention relates to a coating composition for use in forming a thin film having a low refractive index. More particularly, the present invention is concerned with a coating composition comprising a fluid, silica precursor (A) obtained by subjecting a specific alkoxysilane to hydrolysis and polycondensation in the presence of an acid catalyst, a basic compound (B) having a basic hydroxyl group and/or a basic nitrogen atom, wherein the pH of a 0.1N aqueous solution of the basic compound (B) is 11 or more and the vapor pressure of the basic compound (B) is 1.3 kPa or lower as measured at 100° C., and an organic compound (C) having a boiling point of 100° C. or higher, the organic compound (C) being compatible with the fluid, silica precursor (A), wherein the amount of the basic compound (B) is from 0.0015 to 0.5 mol, in terms of the total molar amount of the basic hydroxyl groups and the basic nitrogen atoms in the basic compound (B), per mole of Si atoms contained in the fluid, silica precursor (A). The present invention is also concerned with a porous silica thin film obtained using the above-mentioned coating composition. The porous silica thin film of the present invention has a low refractive index, a high light transmittance and a high strength, so that it can be advantageously used as an antireflection film.
2. Prior Art
Conventionally, as an antireflection film for coating an optical part, a lens (e.g., lenses of glasses) or a display screen, there are known an anti-reflection film having a single-layer structure and an antireflection film having a multilayer structure. An antireflection film having a single-layer structure or a double-layer structure has disadvantageously high reflectance. Therefore, it has been considered to be more desirable to use an antireflection film having a laminated structure composed of three different layers having different refractive indices. However, when such an antireflection film is produced by any of the conventional methods, such as vacuum deposition and dip coating, disadvantages are caused such that the production process is cumbersome and that the productivity is low.
For solving the above-mentioned problems, it has been attempted to reduce the refractive index of a single-layer antireflection film. It is generally known that, when the refractive index of a substrate having formed thereon a single-layer antireflection film is defined as ns and the refractive index of the single-layer antireflection film is defined as n, the minimum value of the reflectance R of the antireflection film is expressed as (ns−n2)2/(ns+n2)2, with the proviso that ns>n. The minimum value of reflectance R (i.e., (ns−n2)2/(ns+n2 )2) is a function which becomes minimum when n2=n2 (i.e., when n=(ns)1/2), which means that the reflectance R becomes smaller as the refractive index n of the single-layer antireflection film becomes closer to (ns)1/2. Materials of conventional light transmitting optical substrates include glass (ns=about 1.52), polymethyl methacrylate (ns=about 1.49), polyethylene terephthalate (ns=about 1.54) and triacetyl cellulose (ns=about 1.49). Accordingly, the appropriate refractive index n of a single-layer antireflection film is within the range of from 1.22 to 1.24. More specifically, for lowering the reflectance R of a single-layer antireflection film formed on the conventional light transmitting optical substrate, it is desired that the refractive index n of the single-layer antireflection film is within the range of from 1.22 to 1.24 or as close to this range as possible.
Silica has an excellent light transmittance over a wide range of wavelength, and has a relatively low refractive index, namely a refractive index of about 1.45. Further, a porous silica has an even lower refractive index. As examples of the method for producing a porous silica thin film having a low refractive index using porous silica particles, the following methods are known.
(1) A method in which porous silica particles are prepared, and the prepared porous silica particles are immobilized on a substrate using a binder, to thereby form a porous silica thin film having a low refractive index on the substrate, in which the silica particles are bonded to the substrate via the binder (Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 3-78946, Hei 6-345487 and Hei 7-48527).
(2) A method in which a thin film comprising a silica particle sol (containing a large amount of silanol groups, and having a relatively low condensation degree) is formed, followed by curing the thin film so as to convert the silica particle sol into solid silica particles, wherein voids are formed between the mutually adjacent solid silica particles (i.e., pores are formed in the thin film) (Unexamined Japanese Patent Application Laid-Open Specification No. 2001-115028).
(3) A method in which a thin film composed of a plurality of components (including porous silica particles) is formed, followed by curing the thin film, wherein pores are formed in the thin film by utilizing the difference between the shrinking ratios of the components (Unexamined Japanese Patent Application Laid-Open Specification Nos. Sho 62-17044, Hei 6-299091 and Hei 8-319109).
However, the above-mentioned methods (1) to (3) have the following problems.
In method (1) above, it is necessary to use a binder to improve the adhesion between the porous silica particles and the adhesion of the porous silica particles to the optical substrate, so as to improve the strength of the porous silica thin film. Due to the use of such a binder, it is impossible to improve the porosity of the porous silica thin film. Therefore, by this method, it is impossible to obtain a porous silica thin film having a low refractive index and a low reflectance.
In method (2) above, for obtaining a porous silica thin film having a desired porosity, it is necessary to cure the thin film of silica particle sol at a high temperature to increase the condensation degree of silica, so as to increase the shrinking ratios of the solid silica particles, thereby increasing the porosity of the thin film to a desired level. Therefore, there is a problem in that, when an optical sheet or film having a low heat distortion temperature is used as a substrate, the curing of the thin film cannot be conducted at a satisfactorily high temperature and, hence, it is impossible to obtain a porous silica film having a satisfactory strength (in terms of the pencil hardness of the thin film and the adhesion strength between the substrate and the thin film).
In method (3) above, as components of the thin film, a linear chain silica polymer produced in the presence of an acid catalyst and porous silica particles produced in the presence of a basic catalyst are used. In method (3), for increasing the porosity of the thin film, it is necessary to increase not only the difference between the shrinking ratios of the components of the thin film, but also the porous silica particle content of the thin film. As a result, scattering of light markedly occurs due to the large amount of porous silica particles, thereby causing a problem that the light transmittance of the thin film is lowered, namely, the haze exceeds 1.
Thus, by the above-mentioned methods (1) to (3), it was impossible to form a porous silica thin film on an optical sheet or film, which porous silica thin film has a low refractive index, a high light transmittance and a high strength.
On the other hand, as an example of the method for producing a porous silica thin film having a low refractive index and a high light transmittance without using porous silica particles, the following method is known.
(4) A method in which a thin film composed of a plurality of components (including an alkoxysilane or a hydrolysis/polycondensation product thereof and at least one pore-forming agent) is formed, followed by extraction of the pore-forming agent from the thin film, thereby forming uniform pores in the thin film, wherein each of the pores has a diameter smaller than the wavelength of a visible light (Unexamined Japanese Patent Application Laid-Open Specification No. Hei 1-312501 (corresponding to U.S. Pat. Nos. 5,116,644 and 5,181,142) and Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 3-199043, Hei 7-140302 and Hei 10-158012).
However, the techniques disclosed in the above-mentioned patent documents (describing the method (4) above) have the following problems.
In the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Hei 3-199043, a coating liquid containing a monoalkyltrialkoxysilane, a polyether and a solvent (mixed solvent containing water, ethanol and 1-butanol) is prepared, and the prepared coating liquid is applied to a substrate to form a thin film on the substrate. Then, the thin film is cured, and the polyether is extracted with ethanol from the cured thin film so as to form pores in the thin film. However, in this document, an inorganic or organic acid is used as a cure accelerating catalyst, so that the thin film cannot be cured to a satisfactory level, and thus, the strength of the thin film becomes unsatisfactory. When the polyether is extracted from such a thin film, the thin film is swollen, and is delaminated from the substrate.
With respect to the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Hei 7-140302, in the first step of the method disclosed in this patent document, a thin film of a coating composition comprising an alkoxysilane is formed on a substrate and, then, the alkoxysilane is subjected to hydrolysis and polycondensation in the absence of a catalyst, so that the resultant thin film contains only a small amount of silica polymers having linear structures. Therefore, in the second step in which the obtained thin film is cured in the presence of a basic catalyst, it is impossible to form a strong three-dimensional silica network in the thin film, so that the strength of the thin film becomes unsatisfactory. Due to the unsatisfactory strength of the thin film, shrinkage of the thin film is likely to occur during the extraction of the pore-forming agent from the thin film and the subsequent drying of the thin film, so that the refractive index of the thin film becomes unsatisfactory.
The present inventors made studies with respect to the method disclosed in the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Hei 7-140302, and have found the following. When a basic catalyst is also used in the first step of the above-mentioned method, where an alkoxysilane is subjected to hydrolysis and polycondensation, the porous silica particles are formed in the thin film. However, in the resultant film, a chain silica polymer is not formed in the thin film. Due to the absence of a chain silica polymer, a uniform silica/pore-forming agent complex is not formed. Therefore, even when the pore-forming agent is extracted from the thin film to form pores in the thin film, the thin film still contains large porous silica particles which cause scattering of the light, thereby leading to the lowering of the light transmittance of the thin film. Further, with respect to the method described this patent document, there is a case where a thin film cannot be formed on an optical substrate because of a precipitation which occurs in the coating composition for forming the thin film (see Comparative Example 8 of the present application).
In the above-mentioned Unexamined Japanese Patent Application Laid-Open Specification No. Hei 10-158012, a method is described in which the hydrolysis and polycondensation of an alkoxysilane are performed in the presence of an acid catalyst, followed by addition of aqueous ammonia as a basic curing catalyst, thereby obtaining a coating composition for forming a porous silica thin film. However, when such a coating composition is coated on an optical sheet or film, so as to form a porous silica thin film on the optical sheet or film, the basic curing catalyst volatilizes from the thin film (i.e., the basic catalyst content of the thin film is lowered) and, hence, the condensation of silica in the thin film would not satisfactorily proceed. Therefore, the resultant porous silica thin film suffers disadvantages that the thin film is delaminated from the optical substrate during the extraction of the pore-forming polymer (organic polymer having an amide bond) from the thin film, and that the thin film gets shrunk after the drying thereof. Thus, by the method of this patent document, it is impossible to form a thin film having a low refractive index, and the obtained thin film has no antireflection effect.
As apparent from the above, by the conventional methods, it has been impossible to obtain a porous silica thin film, which has a low refractive index, a high light transmittance and a high strength. Therefore, there has been a demand for the development of such a porous silica thin film.